The present invention is directed to battery charging methods and apparatuses, and especially to charging battery arrays having a plurality of battery cells. In particular, the present invention is useful in connection with balancing individual cells while charging multiple cell battery arrays, including Lithium-ion or Lithium polymer battery packs.
Many systems use batteries that are configured as battery packs or arrays including a plurality of individual battery cells coupled in series. Such a configuration is commonly encountered, for example in systems that need to maximize run time and use Li-ion (Lithium ion) or Li-polymer chemistry. The battery arrays may include two cells (e.g., for consumer products such as camcorders or cameras) up to four or more cells (e.g., for high-end notebook computers). In multi-cell battery arrays such as Li-ion battery packs with cells arranged in series the overall battery pack coulombmetric capacity is limited by the least capacity cell. As a result, energy capacity of a battery pack is dependent upon how closely individual cell voltages are matched. Cell mismatches of 100 mv (millivolts) can decrease battery pack energy capacity by more than 10%.
Such cell mismatches can be introduced during fabrication or during the processes of charging and discharging the battery array. Cell factory manufacturing can be as closely controlled as to produce capacity differences among cells in a battery array within 50 mv for Li-ion cells. However, cell imbalance or mismatch may be introduced by a number of factors independent of initial factory matching. Those factors contributing to cell imbalance include, by way of example, variations in individual cell chemistry, cell impedance, self discharge rates, capacity fade, operating temperature and other variations among respective individual cells. Cell temperature mismatches are a significant cause of cell mismatching that is a relatively common trait for densely packed products having multiple individual heat sources located close to the battery pack. For example, a 20xc2x0 C. temperature mismatch can cause a voltage differential among cells as high as 100 mv in a charge cycle. One example of such a product is a notebook computer.
Because of the various problems resulting from cell mismatches, cell balancing while charging a battery pack is an important factor in maximizing battery pack energy capacity. Two methods are currently used to balance cells during charging battery packs having multiple cells.
One method presently in use involves differential cell measurement. Using differential cell measurement, individual cell voltages are sampled and differential cell voltages are calculated during charging. When a high differential voltage is detected, charging is interrupted and individual cells are selectively discharged appropriately to obtain balance among cells. This differential cell measurement approach facilitates accurate cell balancing, but complex circuitry and methodology are required to practice the method. For example, it is necessary to use cell voltage translation, A/D (analog-to-digital) conversion and multiple arithmetic operations to practice differential cell measurement. Because of the complexity of the equipment and calculations required for practicing the method, differential cell measurement is usually found to be employed for cell balancing in high-end, high-cost products that include an analog front-end IC (integrated circuit) for measuring voltages in cooperation with a microcontroller or CPU (central processing unit)xe2x80x94based evaluating system.
A second method presently used to balance cells during charging battery packs having multiple cells is a ground referenced, fixed threshold method. Using such a fixed threshold method, when one cell reaches a first predetermined threshold it is discharged to a lower second threshold. The second threshold is usually a fixed threshold set to a voltage equal to or greater than the target voltage, or regulated voltage for the battery pack. The fixed threshold method is less expensive to equip and practice than the differential cell measurement method described above, but it can suffer from low accuracy and can require significantly longer charge times than may be experienced using a differential cell measurement method described above. The likelihood for longer charge times is particularly high if initial voltage mismatch among cells is large.
In both the differential cell measurement method and the fixed threshold method, a charger on the system side must be controlled by the host product it is charging in the battery pack in order to prevent false termination of charging during sampling or cell balancing intervals.
There is a need for a low cost accurate cell balancing method that does not significantly lengthen charge times.
There is a particular need for such a low cost cell balancing method that does not require control by or communication with the host device being charged.
There is also a need for a charge control apparatus that can operate without requiring control by or communication with the host device in the battery pack being charged and can permit cell balancing without causing false termination of charging operations.
A method for controlling voltage levels among cells while charging a battery array to a target voltage uses cell balancing modes employing respective charging currents. The method includes the steps of: (a) In no particular order: (1) establishing a parametric criterion for identifying each cell balancing mode; (2) identifying a performance parameter associated with selected cells for each cell balancing mode; and (3) establishing an exit criterion for each cell balancing mode; (b) ascertaining the onset of charging; (c) identifying the extant cell balancing mode; (d) employing the charging current for the extant cell balancing mode for cell balancing; (e) for selected cells: (1) obtaining an extant parameter associated with the cell; (2) comparing each extant parameter with the exit criterion; and (3) repeating steps (e)(1) through (e)(2) until the extant parameter satisfies the exit criterion; (f) terminating the extant cell balancing mode; (g) repeating steps (c) through (f) until the target voltage is achieved; (h) terminating charging.
It is, therefore, an object of the present invention to provide a system and method for controlling cell balancing of a battery array that is inexpensive, accurate and does not significantly lengthen charge times.
It is a further object of the present invention to provide a system and method for controlling cell balancing of a battery array that can permit cell balancing without causing false termination of charging operations.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention.